The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
Technologies to harness energy from renewable energy sources, such as solar energy, are attracting growing attention due to the increasing awareness of global climate change as a consequence of greenhouse gases emissions from anthropogenic sources, together with the need to mitigate air pollution, the long term resource constraints of fossil fuels and desire to participate in the growing market for renewable energy technologies. Concentrating Solar Thermal (CST) is a rapidly growing technology field owing to its capacity to harness the entire solar spectrum, to achieve high efficiency through achieving high temperatures and because of its good compatibility with conventional thermal energy technologies. The most efficient way to transfer the concentrated solar radiation from the sun to the end product is via direct irradiation of particle “clouds” (i.e. suspensions of closely spaced particles) through an open aperture or through a window, rather than through the walls of a tube. This is because a wall is at a much lower temperature than the sun, as limited by its melting temperature, which reduces the amount of heat transfer. Particles have the additional advantage that they are efficient absorbers of radiation and they are well suited to high temperature storage of thermal energy, which is cheaper and more efficient than is the storage of electrical energy, while energy storage is necessary to mitigate the intermittent availability of solar radiation. Particles can be either inert or reactive, with the latter offering the further advantage that chemical energy carriers are long-term, transportable and dispatchable. The devices used to capture radiation with particles in CST systems are called “particle receivers”.
Solar particle receivers for high-temperature applications generally adopt a cavity-type configuration, both to contain the particles and to control heat losses. Such a configuration comprises a well-insulated enclosure defining a cavity and an opening, or aperture, for an effective capture of the incident concentrated solar radiation. The methods for heating the particulate solid feedstock to high temperatures can be divided in two types: directly irradiated and indirectly irradiated receiver. In the former, the particles are directly irradiated with the concentrated solar radiation, either through a window or an open aperture, while in the latter the heat from solar radiation is transferred by convection and radiation through the reactor walls that receive the radiation. While the working temperature of the indirectly irradiated receiver is limited by the thermal property limitations of the wall absorber, the directly irradiated configuration does not have this limitation, which results in faster heating rates and enhanced kinetics, together with the capacity to achieve higher temperatures.
A directly-irradiated particle receiver can be employed for industrial process heat applications, solar energy including fuels and chemicals production or for heating the working fluid in a power cycle, such as air in a Brayton cycle, for electricity generation. The solid particles can act as a storage and heat transfer medium or as a “substrate” on which chemical reactions can occur.
One concept of solid particle receiver first proposed in the 1980s called the “falling particle cloud receiver”, was developed for thermal and thermochemical applications. This receiver is usually mounted atop a tower to capture the focused solar energy from a field of heliostats. Sand-size (100-1000 μm) ceramic particles fall down freely into a solar receiver, forming a curtain that directly absorbs the concentrated solar radiation that passes through an aperture. Once heated, the particles can be collected in an insulated tank and/or used to heat a secondary working fluid (e.g. air, steam, CO2). Since the solar energy is directly absorbed inside the inert particles, the heat flux limitations associated with other solar receivers (e.g. tubular central receivers) are avoided and high temperatures (above 1000° C.) can be reached.
However, despite its benefits, the falling particle receiver configuration encompasses several disadvantages. In particular:                a) It does not have a means to control the particle residence time within the cavity as a function of particle size. Several studies have shown that the cavity efficiency and the particle outlet temperature decrease as the particle size increases. This is due to the fact that larger particles need a longer residence time than do smaller ones owing to their higher thermal capacity. Indeed, an increase of the particle size leads to an increase of the particle vertical velocity (larger particles fall faster than smaller particles), thereby reducing the residence time of large particle inside the receiver. Hence, this process is most efficient for mono-disperse particles, which adds significantly to the cost.        b) The particle residence time within the cavity is short. Methods for increasing the temperature of the particles include the use of recirculation and other means to increase the residence time of the particles within the receiver, such as obstructions, inclined plates, porous structures, spiral and centrifugal receivers. However, physical components in high temperature environments reduce the reliability of the system. In addition, these methods are not able to control the particle residence time within the cavity as a function of particle size.        c) The impact of the particles with the collection hopper, structures, or other particles, cause abrasion, degradation and attrition of the particles, so that the particle receiver must operate with a wide range of particle sizes. This leads to a decrease of the process efficiency for the reasons reported above.        
In an alternative device, solid particles can also be adopted to improve the efficiency of conventional volumetric receiver. A volumetric receiver utilises a block with large internal surface area, such as honey-comb, to absorb the radiation and transfer the heat to the surrounding fluid by convection. One study reported an experimental evaluation of a small particle air receiver (10 KW) in which the working gas was seeded with sub-micrometre carbon particles to absorb the radiation. These particles were found to be effecting in increasing to heat transfer to allow very high temperatures (of up to 1800° C.) to be reached at the receiver outlet. Additionally, the particle cloud partially shields the receiver cavity's wall from the concentrated irradiation, reducing some of the material and structural limitations associated with conventional volumetric receivers. However, this device presents several challenges:                a) It requires the development of a solid/gas suspension system that maintains a uniform and high concentration of particles in the receiver since the scattering properties and the residence time of the particles vary significantly with the particle size;        b) It requires the use of sub-micron particles, which are difficult to separate from the fluid at the outlet of the receiver;        c) It does not address the problem of avoiding particle deposition on a window. Since the main application for volumetric receivers is in gas turbines, it is necessary to pressurise the air before heating. This requires a window. No method to prevent this deposition has been proposed.        
Particle receiver-reactors perform an additional process of reaction over particle receivers, so that the particles undergo both heat absorption and chemical conversion. A solar receiver-reactor is invariably utilised to drive an endothermic reaction and so requires additional residence time for the reaction in addition to the sensible heating. In addition, it and must also provide good mixing of the reactants and good sealing from the ambient air to avoid unwanted reactions. Several types of particle receiver-reactor have been developed, to provide different ranges of residence time within the receiver and/or convey different carrier fluids and reactants, together with the solid particles. According to several studies, solar particle receiver-reactors can be categorized in packed bed, fluidized bed and entrained flow reactors. Each group has its characteristic particle size, residence time and operating temperature. This classification is also applicable to solid particle receivers where inert particles are used. In addition, it is important to note that for each reactor group analysed, the heating of inert particles has the same basic issues of chemically reacting particles, although the details are different.
The processing requirement for particles and the selection of the particle receiver fundamentally depend both on the particle size and on their other characteristics, such as density, porosity and reaction time. In particular the particle size influences significantly the heat and mass transfer inside the particle. These can be described by dimensionless numbers. The temperature uniformity within a particle are characterized by the Biot number, Bi, defined as:
      Bi    =                  hd        p                    k        s              ,where h is the external heat transfer coefficient, ks the thermal conductivity of the solid particle and dp the particle diameter. For small Bi (<<1), the temperature is uniform inside the whole volume of the particle while for large Bi (>>1) a steep temperature gradients exists inside the particle. These gradients significantly affect the heat transfer behaviour to a particle and thus the overall heating process. It is important to note that Bi increases linearly with the particle size. A literature review suggest that most entrained solar reactors are characterized by very small Bi, while higher values can be found for packed and fluidized bed.
Similarly to the heat transfer, the mass transfer inside a particle can be described by the Sherwood number, Sh, where Sh is defined as:
      Sh    =                            h          m                ⁢                  d          p                    D        ,where hm is the gas-to-particle mass transfer coefficient and D is the mass diffusivity of the fluid. As highlighted for Bi, the Sh also depends on the particle size and it increases linearly with the particle size. This implies that the mass transfer behaviour inside a particle significantly varies with the particle diameter.
Entrained flow solar reactors operate at elevated temperatures, usually above 1500 K, to compensate for their relatively short residence time. The particles need to be micron-sized (generally 1-200 μm) for them to be kept in suspension, i.e. “entrained”, heated and/or fully react within the reactor cavity by directly absorbing the solar radiation that enters the cavity through the aperture. A vortex flow is employed in all known solar entrained flow reactors to keep the majority of particles away from the aperture, to increase the particle residence time and augment the solar absorption by keeping the particles near the wall. Hence, these reactors are also referred to solar vortex flow reactors (SVR).
In comparison with SVRs, fluidized beds provide a much higher volumetric loading, increased particle residence time and convective mixing between the particle and the gas. In directly-irradiated solar fluidized beds, the bed is directly irradiated by concentrated solar radiation and the particles are semi-suspended in turbulent motion by the working fluid until they are fully reacted or reach a small enough size to be elutriated and carried out from the bed by the fluid. The distribution of temperature within fluidised beds is highly uniform due to the high heat and mass transfer rates of fluidised beds. Operating working conditions depend on the selection of fluidized solid particle type and size and fluidization regime. Operation is limited by the minimum fluidization velocity, i.e. the minimum velocity required to fluidize the bed, which also entails a pressure drop. The details of the minimum velocity depend upon a number of factors, including the shape, size, density, and poly-dispersity of the particles. Due to their characteristics, fluidized beds can process larger particles in comparison to SVRs (order of mm) while working temperatures are generally lower than SVRs (800-850° C.). However, fluidised bed reactors are typically tall and narrow, so that the surface area of the top of the bed is relatively low. For this reason, the heat transfer to the bed in a solar fluidised bed is limited by this surface area. A known example of directly-irradiated fluidized bed solar reactor with a windowed aperture is of the type having an internally circulating fluidized bed solar gasifier. In such an arrangement, coke particles (particle size range was set to 75-710 μm) were gasified successfully using CO2 as gasifier agent at temperatures below 900° C.
Packed bed solar reactors generally have the largest particles, which require the longest particle residence time and achieve the lowest operating temperature of the three groups. The working fluid, usually steam or CO2, is passed through a packed bed of reactant materials either from above or below while the solid particles are heated by the solar radiation. Although high chemical storage efficiencies can be obtained in these type of reactors, for industrially scaling up, the fixed bed reactor has several technical drawbacks, such as limitations of heat and mass transfer, long residence time and the addition of new bed materials is more difficult.
Although these solar reactor concepts are different from each other, they exhibit some common key issues. In particular:                a) In entrained flow and packed bed reactors the particle residence time distribution is approximately independent of particle size. This limitation negatively affects either the size of the reactor or the chemical conversion of large particles and the reactor efficiency because the larger particles require longer residence time (typically by a factor that scales with the diameter ratio to the 3rd power) than do the smaller ones to heat up and/or achieve complete conversion. As a consequence, for a reactor scaled for smaller particles, the large particles are under-processed resulting in a lower particle temperature and overall solar-to-chemical conversion efficiency. Alternatively, if the reactor is sized for the large particles, it needs to be over-sized for the small particles, resulting in higher cost and a higher average temperature in the reactor, which in turn leads to higher radiation losses through the aperture. In fluidised bed reactors, instead, the particle residence time distribution depends on particle size. Particles are retained in the bed until they reach the elutriation diameter. Smaller particles are more likely to be elutriated due to their smaller terminal velocity. A high elutriation rate translates to a shorter particle residence time, which in turn lowers conversion;        b) These receiver-reactors operate with a wide range of particle sizes. This is firstly because the majority of particle generators (e.g. mills, grinders) generate a wide size distribution and, secondly, because particle reactors generate particle breakage and attrition, which breaks the original particles into smaller sizes. Thirdly, many types of chemical reactors consume particles as they are reacted, so that a particle shrinks through the processing stages. However, current receiver-reactors are not designed to optimally accommodate particles of different sizes. Furthermore, the number of large particles within the cavity is relatively small, while the number of fine particles is relatively large owing to the cubic dependence of particle mass on particle diameter;        c) The reactor window is a critical part of the reactor. Although, this has the advantage of reducing the radiation losses it is vulnerable to particle deposition. Any particle deposition poses a great challenge to the reliable operation of the directly-irradiated particle receiver technology because it reduces the solar efficiency and causes localised heating of the window, which in turn leads to potential failure of the system.        
In summary, these issues collectively mean that large particles require a greater residence time within a particle receiver-reactor to be fully processed than do smaller particles. If a reactor is designed so that all particles have a similar residence time, then size of the reactor depends on the size of the largest particle, so the reactor is over-sized for the average particle and becomes very much larger and more expensive than it could be if particle residence time can be controlled as a function of particle diameter. In contrast, a reactor can be smaller and less expensive it if can be designed to provide a longer residence time for larger particles than for smaller particles. This will also lower the average temperature of particles in the reactor, so reducing radiation losses through the aperture.
These issues then generate the need to be able to provide a means to control the particle residence time distribution (RTD) as a function of both particle size and reaction time. That is, the optimum ratio of residence time of large to small particles will vary for different processes. Present solar particle receiver-reactors do not provide any means to either control residence time as a function of particle diameter and/or of reaction time.
The CST for the thermochemical production of solar fuels uses a concentrated solar radiation as energy source to provide the high-temperature process heat needed to drive endothermic chemical reactions, offering a viable path for fossil fuel decarbonisation in the energy sector. Among the available methods for solar fuel production, of particular interest is the steam-based Solar Gasification of carbonaceous solid materials such as coal, biomass or waste materials, which can be used to convert these feedstock materials into high-quality synthesis gas, mainly H2 and CO, usable for power generation in efficient combined cycles and fuel cells, or to produce liquid fuels in the Fischer-Tropsch process. The advantages of using solar energy instead of auto-thermal reactions to provide the process heat are numerous. These include an upgraded to the calorific value of the carbonaceous feedstock, a higher H2 to CO ratio in the product syngas and reduced pollutants discharge due to the lack of combustion. However, this technology is currently more expensive than conventional gasification, so further innovations are required to lower its cost.
Of all directly irradiated solar receiver/reactors, the solar vortex reactor (SVR) has been found to be one of the most promising concepts, being applied successfully to the steam gasification of petroleum coke powder, coke-water slurry and liquefied vacuum residue.
Several examples of SVRs are disclosed in U.S. Pat. Nos. 7,024,857, 7,449,158, and 8,257,454.
Current design of the SVR consists of a cylindrical cavity with a windowed aperture and a compound parabolic concentrator (CPC). Particles are injected into the reactor through tangential inlets to generate a vortex flow within the reactor, which transport the particles through the reactor and achieve effective absorption of the concentrated solar radiation. Typical reactor temperatures are in the range 1300-1800 K, that are common for many thermochemical reactions. Nevertheless, despite its benefits, current design also suffers from the following disadvantages that need to be addressed further. In particular:                a) Large particles are under-processed relative to small particles. As mentioned above, this is a consequence of the fact that particle residence time distribution within the reactor is independent of particle size. Note that the mass fraction of the larger particles is significant, even though their number is relatively low.        b) The particle residence time within the cavity is relatively short, so that very high working temperature (above 1400 K) and high-reactivity feedstocks are needed to compensate this drawback. Although the residence time will increase with scale, it is nevertheless desirable to increase the residence time of the largest particles at each scale;        c) The SVR suffers from particle deposition on the reactor window. Current mitigation strategies employ auxiliary gas jets configured to generate a “curtain” of clean gas that seek to mitigate particle deposition onto the window surface. However, the use of auxiliary gas inlets decreases the efficiency of the process significantly since the required mass-rate of purging gas is sufficiently great as to constitute a significant parasitic loss of sensible heat. This strategy also increases the overall costs of the process, both in CapEx and OpEx. Finally, the optimal configuration of the purging gas nozzles depends upon the fluid-dynamic flow structure established within the cavity, so that their use tends to reduce operational flexibility and limit operation to a more restricted range of working conditions.        d) The state-of-the-art in SVR configurations employs a window. This has both advantages and disadvantages. It allows control of the atmosphere in the cavity, allows moderate pressurisation and avoids local pollutant emission by preventing the ingress and egress of gases and particles through it. It also reduces radiation losses, since it is opaque to the longer radiation wavelengths. A window also allows a certain amount of pressurisation, which has advantages for some applications. However, it is also usually expensive and limits the maximum size of the reactor, owing to the manufacturing constraints of producing large windows. In addition, it reduces the solar energy absorption efficiency, particularly in the case of any particle deposition, and is vulnerable to breakage. A window-less reactor, if available in a configuration that avoids significant ingress and egress, is likely to be the preferable alternative for a number of applications, although no window-less configurations has previously been proposed.        
It is against this background, and the problems and difficulties associated therewith, that the present invention has been developed. While the present invention was developed against this background, it need not necessarily overcome any or all of the problems and difficulties referred to above. Rather, the invention may merely offer an alternative arrangement for exposing heat absorbing particles to concentrated solar radiation.